The present invention relates to kilns for firing ceramic products such as brick, sewer pipe, refractories and the like.
In the manufacture of such ceramic products, it is necessary to fire the molded clay ware in a kiln at high temperature for a substantial period of time. Several types of kilns have been used for this purpose. The type of kiln known as a shuttle kiln consists generally of a rectangular refractory enclosure provided with burners for heating the ware which is stacked on cars for movement into and out of the kiln. Kilns of this type have the great advantage of flexibility in use but in the past have had the disadvantages of high capital cost and high fuel costs as compared to other types of kilns. This was due primarily to the usual construction of such kilns in which the hot gases from the burners flowed upward through the ware and were exhausted at the top, making only a single pass over the ware, so that large amounts of heat were wasted and the fuel costs were correspondingly high. Earlier attempts at building kilns for downdraft operation, in which the gases flow downwards through the ware, involved quite complicated and expensive constructions and such kilns were not widely used.
In my U.S. Pat. No. 3,854,865, there is disclosed a new type of shuttle kiln of relatively simple design which uses downdraft gas flow with recirculation and can be operated at low fuel cost. The kiln of this prior patent consists of a generally rectangular refractory enclosure with a movable floor consisting of one or more cars for supporting the ware and moving it into and out of the kiln. Burners are provided in the upper part of the kiln which are supplied with excess air to provide large volumes of high velocity heated gas in the top of the kiln which is directed to flow downward through the ware. The gas exhausts laterally through the floor of the kiln into a system of ducts in the side walls which direct the gas upward toward the burners. The gas is there aspirated into the gas flow from the burners and mixes therewith for recirculation through the kiln. A very effective downdraft gas flow is thus obtained which results in relatively low fuel cost. This prior construction, however, requires essentially double walls to provide the necessary ducts and flues for circulation of the gas, and thus involves a relatively large amount of brickwork with attendant high cost.